Fault-size dependent fracture energy and multi-scale earthquake rupture cascades

Alice-Agnes Gabriel, Dmitry I. Garagash, Kadek H. Palgunadi, & Paul Martin Mai

Published September 11, 2022, SCEC Contribution #12509, 2022 SCEC Annual Meeting Poster #169

Faults and fractures that host dynamic slip are known to vary over more than six orders of magnitude in source dimensions. Connected multi-fault systems on large (e.g., the Ridgecrest fault system) and small (e.g., fracture networks and damage zones in geo-reservoirs) spatial scales may drive dynamic rupture cascades. Frictional fracture energy Gc and the related critical slip distance Dc, over which coseismic fault weakening takes place, control earthquake nucleation and dynamic slip evolution. Previous inferences show that Gc and Dc both increase with earthquake size (e.g. Viesca & Garagash, 2015; Mai et al, 2006; Abercrombie & Rice, 2005), which can be explained by coseismic weakening due to thermal pressurization of pore fluids driven by the rapid shear heating of saturated fault gouge.

Here we explore the possibility that not all earthquakes experience prominent thermal pressurization. Fracture energy can be defined by the breakdown of fault friction, which has a well-defined limit in terms of Dc. Then, the seismologically-inferred increase of fracture energy with event size corresponds to a physically-plausible scaling of Dc with fault size. We recover such scaling from existing compilations of seismologically-inferred fracture energy, which we correct for dynamic undershoot using a model of a circular rupture driven by flash-heated rate-and-state dependent friction. We show that both, the lower-bound Dc and Gc, scale linearly with source size in the range from 10 m to 10 km.

Our inferred Dc vs. fault/fracture size scaling provides an intuitive explanation of seismicity being observed on all scales, including the possibility of small, volumetric earthquakes hosted completely off main faults, e.g., across subsidiary fractures in the damage zone. This scaling also supports mechanical viable composite earthquake ruptures occurring as a cascade over networks of faults of very different sizes. In a numerical example, we apply this scaling to drive 3D dynamic earthquake rupture as a cascade over a network of several hundred fractures in a damage zone with or without activation of an embedded main (misoriented) fault.

Citation
Gabriel, A., Garagash, D. I., Palgunadi, K. H., & Mai, P. (2022, 09). Fault-size dependent fracture energy and multi-scale earthquake rupture cascades. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)